1. Field of the Invention
The present invention relates to a semiconductor device formed by using a semiconductor film having a crystalline structure (crystalline semiconductor film), and to a method of manufacturing the semiconductor device. The present invention particularly relates to a semiconductor device containing a field effect transistor in which a channel formation region is formed by a crystalline semiconductor film formed on an insulating surface, and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, a technique of forming a thin film transistor (TFT) over a substrate has greatly progressed, and its application and development for active matrix semiconductor display devices have been advanced. A thin film transistor (hereinafter referred to as TFT) which is manufactured by using a semiconductor film having a crystalline structure has been applied to flat panel displays represented by a liquid crystal display device. In particular, since a TFT using a polycrystalline semiconductor film has higher field-effect mobility (also referred to as mobility) than a TFT using a conventional amorphous semiconductor film, it enables high-speed operation. It is therefore possible to control the pixel by the driver circuit formed over the same substrate where the pixel is formed, though the pixel is conventionally controlled by a driver circuit provided outside the substrate.
For substrates used in semiconductor devices, a glass substrate is regarded as promising in comparison with a single crystal silicon substrate in terms of the cost. A glass substrate is inferior in heat resistance and is easily subjected to thermal deformation. Therefore, a technique of forming an amorphous silicon film on an insulating substrate made of glass or the like and using laser processing (laser annealing) for crystallization is known. Therefore, in the case where a polysilicon TFT is formed over the glass substrate, in order to avoid thermal deformation of the glass substrate, the use of laser annealing for crystallization of the semiconductor film is extremely effective.
The term “laser annealing” in the semiconductor manufacturing process indicates a technique for re-crystallizing a damaged layer formed on a semiconductor substrate or on a semiconductor film and a technique for crystallizing a semiconductor film formed on an insulating surface. This also includes a technique that is applied to leveling or improvement of a surface quality of the semiconductor substrate or the semiconductor film. Laser oscillation apparatuses used for laser annealing are: gas laser oscillation apparatuses represented by an excimer laser; and solid state laser oscillation apparatuses represented by a YAG laser. It is known that such a device performs crystallization by heating a surface layer of the semiconductor by irradiation of the laser light in an extremely short period of time of about several tens of nanoseconds to several tens of microseconds.
Laser annealing has characteristics such as remarkable reduction of processing time compared to an annealing method utilizing radiant heating or thermal conductive heating. In addition, a semiconductor or a semiconductor film is selectively and locally heated so that a substrate is scarcely thermally damaged.
Lasers are roughly divided into two types: pulse wave oscillation and continuous wave oscillation, according to an oscillation method. In the pulse wave oscillation laser, an output energy is relatively high, so that mass productivity can be increased by setting the size of a laser beam to several cm2 or more. In particular, when the shape of the laser beam is processed using an optical system and made to be a linear shape of 10 cm or more in length, it is possible to efficiently perform irradiation of the laser light to the substrate and further enhance the mass productivity. Thus, for crystallization of the semiconductor film, the use of a pulse wave oscillation laser is becoming mainstream.
In recent years, however, it has been found that the grain size of crystals formed in a semiconductor film is larger when a continuous wave oscillation laser is used to crystallize a semiconductor film than when a pulse wave oscillation laser is used. With crystals of larger grain size in a semiconductor film, the mobility of TFTs formed from this semiconductor film is increased. As a result, continuous wave oscillation lasers are now suddenly attracting attention.
One example of crystallizing an amorphous semiconductor film by using laser light irradiation is a technique, disclosed in JP 62-104117 A (p. 92), in which laser light is scanned at a high speed, equal to or greater than the beam spot diameter×5000/sec, thus performing multi-crystallization of the amorphous semiconductor film without reaching a state in which the amorphous semiconductor film is completely melted. A technique in which drawn out laser light is irradiated to a semiconductor film formed in an island-like shape, substantially forming a single crystal region is disclosed in U.S. Pat. No. 4,330,363 (FIG. 4). Alternatively, a method is known in which a beam is processed into a linear shape by an optical system and then irradiated like a laser irradiating apparatus disclosed in JP 8-195357 A (pp. 3–4, FIGS. 1–5).
In addition, a technique of manufacturing a TFT in which the second harmonic of laser light is irradiated to an amorphous semiconductor film using a solid state laser oscillation apparatus, such as an Nd:YVO4 laser, thus forming a crystalline semiconductor film having a large crystal grain size compared to a conventionally formed crystalline semiconductor film, is disclosed in JP 2001-144027 A (p. 4).
Incidentally, tests of forming a single crystal semiconductor film on an insulating surface have been carried out from long ago, and a technique referred to as graphoepitaxy has been proposed as a very progressive test. Graphoepitaxy is a technique in which a step is formed on the surface of a quartz substrate, an amorphous semiconductor film or a polycrystalline semiconductor film is formed over the quartz substrate, heat treatment is then performed by using a laser beam or a heater, thus forming an epitaxial growth layer with the step shape formed over the quartz substrate taken as a nucleus. This technique is disclosed in following Document 1 and the like, for example.
Document 1: J. Vac. Sci. Technol., “Grapho-Epitaxy of Silicon on Fused Silica Using Surface Micropatterns and Laser Crystallization”, 16(6), 1979, pp. 1640–3.
Further, a semiconductor film crystallization technique referred to as graphoepitaxy is also disclosed in following Document 2, for example. This is a technique in which epitaxial growth of a semiconductor film by the introduction of an artificially formed surface relief grating on an amorphous substrate surface was tested. The graphoepitaxy technique disclosed in Document 2 is one in which a step is formed on the surface of an insulating film, and epitaxial growth of a semiconductor film crystal is attained by conducting heat treatment, laser light irradiation, or other processing on a semiconductor film formed on the insulating film.
Document 2: M. W. Geis, et al., “CRYSTALLINE SILICON ON INSULATORS BY GRAPHOEPITAXY” Technical Digest of International Electron Devices Meeting, 1979, p. 210.
However, when a laser light is irradiated to an amorphous semiconductor film formed over a flat surface for crystallization, crystals become polycrystals, thereby arbitrarily forming defects such as a grain boundaries, so that crystals with aligned orientation cannot be obtained. Differing from the crystal grains, countless carrier traps such as recombination centers and capture centers exist in the grain boundaries due to an amorphous structure, crystal defects, and the like. It is known that when a carrier is trapped in the carrier trap, the potential of the grain boundaries rises, and the grain boundaries become impediments with respect to the carrier such as an electron or a hole, and therefore the current transporting characteristics (mobility) for the carrier are reduced.
The existence of grain boundaries within the TFT active layer, in particular within a channel formation region, therefore exerts a great influence on the characteristics of the TFT in which the TFT mobility drops considerably, the ON current is reduced, and the OFF current is increased due to electric current flowing in the grain boundaries. Further, the characteristics of a plurality of TFTs, manufactured on the premise that the same characteristics can be obtained, may vary depending on the existence of grain boundaries within the active layers.
The position and size of the crystal grains obtained when irradiating laser light to a semiconductor film become random due to the following reasons. A certain amount of time is required until the generation of solid state nuclei within a liquid semiconductor film which is completely melted by the irradiation of laser light. Countless crystal nuclei are generated in completely melted regions along with the passage of time, and crystals grow from each of the crystal nuclei. The positions at which the crystal nuclei are generated are random, and therefore the crystal nuclei are distributed non-uniformly. Crystal growth stops at points where the crystal nuclei run into each other, and therefore the position and the size of the crystal grains become random.
A crystalline semiconductor film manufactured by using a laser annealing method, which is roughly divided into pulse wave oscillation and continuous wave oscillation types, is generally formed with an aggregation of a plurality of crystal grains. The position and size of the crystal grains are random, and it is difficult to specify the crystal grain position and size when forming a crystalline semiconductor film. Crystal grain interfaces (grain boundaries) therefore exist within an active layer formed by patterning the aforementioned crystalline semiconductor film into an island-like shape.
In the flat panel displays described above, a semiconductor film is formed on a glass substrate to build a TFT and the TFT is arranged giving no regard to randomly-formed grain boundaries. Therefore the crystallinity of a channel formation region of the TFT cannot be controlled strictly. The randomly-formed grain boundaries and crystal defects degrade the characteristics and cause fluctuation in characteristics between elements.
It is ideal to form the channel formation region, which exerts a great influence on the TFT characteristics, by a single crystal grain, thus eliminating the influence of grain boundaries. However, it is nearly impossible to form an amorphous silicon film, in which grain boundaries do not exist, by using a laser annealing method. It has therefore not been possible to date to obtain characteristics equivalent to those of a MOS transistor, which is manufactured on a single crystal silicon substrate, in a TFT that uses a crystalline silicon film crystallized by employing laser annealing.
Therefore, a method of recrystallization from a melted state by heating a semiconductor film on an insulating substrate to a high temperature, known as zone melting and the like, is in the mainstream in order to form a high quality crystalline semiconductor film on an insulating surface having aligned orientation, few defects, few crystal grain boundaries, and few subboundaries.
However, a single crystal semiconductor film cannot be formed on a glass substrate having a relatively low distortion point even though a known graphoepitaxy technique, which is one of zone melting methods is used.
In any case, it has not been possible to form a crystalline semiconductor film in which semiconductor volumetric contraction caused by crystallization, defects due to thermal stresses with the base, lattice mismatching, or the like, crystal grain boundaries, and subboundaries do not exist. Except a bonded SOI (silicon on insulator), it has not been possible to obtain quality equivalent to that of a MOS transistor formed on a single crystal substrate, in a crystalline semiconductor film that is formed on an insulating surface and then crystallized or recrystallized.